During an immune response, naIve B cells often generate germinal centers in secondary lymphoid organs. In this specialized environment, B-lymphocytes proliferate extensively and their antibody molecules undergo two dramatic genetic changes: their affinity for the antigen is increased by somatic hypermutation and their constant region (CH) genes are replaced for downstream isotypes by class switch recombination. The molecular mechanisms that generate switching and hypermutation are mostly unknown, however both reactions are abrogated in mice or humans deficient on AID, a recently characterized B cell molecule of unknown function, and DNA double-stranded breaks (DSBs) are thought to be intermediate in both processes. It has also been argued that DNA DSBs occurring at the Ig loci of germinal center B cells are involved in chromosomal translocations and tumorigenesis in humans and mice. For instance, in susceptible BALB/c animals, which develop plasmacytomas upon repeated injections of pristane oil, virtually all induced tumors carry chromosomal translocations juxtaposing the c-myc gene to the heavy chain or light chain locus. Whether these translocations are by-products of hypermutation or switching has not been established. The primary aim of this proposal is to determine the exact role of recombination and hypermutation in the pathology of B cell malignancy. To accomplish this goal, we will study plasmacytomagenesis in susceptible BALB/c mice that are either deficient for AID or ectopically express AID. These studies will investigate whether switching and hypermutation are required to give rise to translocations and B cell neoplasia. To define how DNA lesions might be aberrantly targeted to non-lg genes we will define the DNA sequences that are critical to induce hypermutation and switching in Ig genes. To investigate which DNA repair pathways rejoin the translocating chromosomes, we will study plasmacytomagenesis in animals deficient for a variety of DNA repair factors. Our working hypothesis is that the B cell genomic instability leading to tumorigenesis is the result of abnormal switch recombination and somatic hypermutation.